The present invention relates to a sealing method for connecting portions of heat-seal dielectric sheet material.
The present invention is particularly advantageous for use on overwrapping machines, particularly machines for cellophaning packets or cartons of cigarettes, to which the following description refers purely by way of example.
On cellophaning machines of the above type, a sheet of transparent synthetic heat-seal material, normally polypropylene, is folded into a U about a respective product comprising a carton or packet of cigarettes, so that two opposite portions of the sheet project beyond the product; the two portions are then folded one on top of the other and onto an outer surface of the product to form a tubular wrapping; and the wrapping is stabilized by heat-sealing the two portions together by means of heat applied, for example, by a heat-sealing device or any other external heat source.
On known cellophaning machines, heat-sealing the two portions together normally poses serious difficulties, mainly due to them not adhering perfectly to each other during heat-sealing. The presence, in fact, of even only relatively small air pockets between the two portions impairs heat transmission between them, so that hotter sources than strictly necessary must be used, and which may easily result in singeing of the portions in the event of even only a slight variation in exposure time. Moreover, as it is expanded by the heat transmitted during sealing, the air inside the pockets may cause the heat-seal portion to burst, thus detaching the portions one from the other.